Buckle up, California. Some serious 'precipitation whiplash' predicted for the state

Sacramento area flooding through the years: 1862-2017

Video takes a quick glance at flooding during several years beginning in 1862.
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Video takes a quick glance at flooding during several years beginning in 1862.
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It was the greatest flood in recorded California history, 43 days of rain and snow that swamped the state, killed thousands of people and forced the newly elected governor to take a boat to his inauguration at the Capitol.

Now a group of climatologists says global warming will increase California's risk to repeat performances of the devastating flood of 1862.

In a study published Monday in the scientific journal Nature Climate Change, the scientists say climate change will increasingly expose California to a phenomenon they call "precipitation whiplash," in which drought or drought-like conditions will alternate with intensely rainy winters. Rain and snow will become concentrated in narrow windows of time at the peak of winter, instead of being spread between October and April.

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"The already distinct contrast between California's long, dry summers and relatively brief, wet winters will probably become even more pronounced," the scientists wrote.

When scientists discuss climate change's impact on California, they usually talk about worsening droughts: Warmer winters will diminish the Sierra Nevada snowpack, the argument goes, making it harder to store precipitation for the summer months that follow. Hotter summers will put additional strains on water supplies.

The Nature Climate Change article shines an uncomfortable spotlight on California's flood risk from climate change. "In some circles the increasing risk of flood could have been overlooked to date," said UCLA climatologist Daniel Swain, a co-author of the article.

The region has spent $2 billion in the past 20 years to strengthen levees along the Sacramento and American rivers, and reduce other vulnerabilities, but the risk remains significant. Portions of the region still lack 100-year flood protection, including 25 percent of the city of Sacramento. That means those areas couldn't withstand a storm with a 1-in-100 chance of occurring in a given year, and property owners in those areas generally are required to buy flood insurance.

The Sacramento Area Flood Control Agency, or SAFCA, says it is committed to achieving a minimum of 200-year flood protection, the threshold established by the Legislature in 2007. A total of $2.4 billion worth of projects is in the pipeline, including work on levees around Natomas and Arcade Creek that's scheduled to begin later this year. SAFCA also is working with federal officials on a plan to raise Folsom Dam, which could push the region's flood protection to as high as 300 years.

Yet much of the work on SAFCA's to-do list is years from completion. And even 300-year flood protection might not be enough. Experts say a mega-flood similar to 1862 could overwhelm even the most advanced levees and other defenses. The U.S. Geological Survey, in a 2011 study, said the 1862 disaster was probably a 500- or 1,000-year storm.

The 1862 storm was pure catastrophe. "Drowning deaths occurred every day on the Feather, Yuba and American rivers," the magazine Scientific American recalled in a 2013 article. "In one tragic account, an entire settlement of Chinese miners was drowned by floods on the Yuba River." The water was 30 feet deep in some places, and Gov. Leland Stanford took a rowboat from the governor's mansion to his inauguration.

Although flood control obviously has improved since the 19th century, there are now millions of people and billions of dollars of property in close proximity to levees and reservoirs. The Geological Survey's study said a storm akin to 1862 could cause $400 billion in property damage statewide and another $325 billion in business-interruption expense.

"We're absolutely not ready for a storm that size," said Jeffrey Mount, a water expert at the Public Policy Institute of California. Mount didn't contribute to the Nature Climate Change article but is familiar with its findings.

State officials are starting to take a new approach to flood safety that could help California withstand monster storms. Last year the Central Valley Flood Protection Board, an arm of the Department of Water Resources, released a blueprint that relies less on strengthening levees to tame the rivers and more on resisting housing development in the worst of the high-risk zones.

Arguing that California must give its rivers more room to roam, the DWR report said too many residents are "still at unacceptably high risk from catastrophic flooding."

Barry Nelson, a Bay Area water policy consultant, said the new plan is a breakthrough in flood control but isn't an immediate fix, either. "Preparing to manage our flood system and our natural resources is kind of like planning for retirement," he said. "You can't wait until you're 64. It's going to take decades."

In the meantime, Swain said the rising risk of mega-flooding, including the chance of a disaster along the lines of 1862, should alarm every Californian.

"It goes from being something that might have happened once every other century, essentially, to happening maybe multiple times over the next 80 years," Swain said in an interview. "We go from a situation where it would be such an outlier that we probably don't have to deal with it ... to one where it becomes something that we almost certainly will have to deal with, and probably sooner rather than later."

Why would climate change make for more intense rainstorms? Because warmer temperatures increase the atmosphere's capacity to hold water vapor during the rainy season, he said. That doesn't necessarily mean every storm will be heavier, but it increases the likelihood.